1
Classe di Scienze Matematiche, Fisiche e Naturali
Corso di Perfezionamento in Scienze Chimiche
“The chemistry of unsaturated hydrocarbyl
ligands bridging coordinated in
diiron or diruthenium complexes”
Adriano Boni
2 Index
1. Abstract...3
2. Introduction...5
2.1 Allenyl ligand: types of coordination………...7
2.2 Synthesis of dinuclear µµµµ−−−allenyl complexes………..13 −
2.3 Chemistry of dinuclear µµµµ−−−allenyl complexes………17 −
2.4 Vinyl ligand: types of coordination...25
2.5 Synthesis of dinuclear µµµ−µ−−−vinyl complexes……….31
2.6 Chemistry of dinuclear µµµµ−−−−vinyl complexes………...36
2.7 Objective...42
3. Results and Discussion...43
3.1 Dimetallacyclopentenones: Synthesis and characterization...43
3.2 Chemistry of the Cationic diruthenium µµµµ−−−−allenyl complexes………...47
3.3 Chemistry of the cationic diiron µµµµ−−−−allenyl complexes……….69
3.4 Chemistry of the cationic diriron µµµ−µ−−−vinyl complexes………...78
4. Conclusions...102
5. Experimental...106
6. Crystallographic Appendix...120
3
1. Abstract
The reactivity of the previously reported diruthenium µ-allenyl complex [Ru2(Cp)2(CO)2(µ-CO){µ-η1:η2α,β-C(H)=C=C(Me)2}][BF4] (4a) and of the new one
[Ru2(Cp)2(CO)2(µ-CO){µ-η1:η2α,β-C(H)=C=C(Ph)2}][BF4] (4b) have been investigated.
The reaction of 4b with Brönsted base results in formation of the µ-allenylidene derivative [Ru2Cp2(CO)2(µ-CO){µ-η1:η1-C=C=C(Ph)2}] (10). The nitrile adducts
[Ru2Cp2(CO)(NCMe)(µ-CO){µ-η1:η2-C(H)=C=C(R)2}]+ (R = Me, 6a; R = Ph, 6b),
prepared by treatment of 4a,b with MeCN/Me3NO, react with N2CHCO2Et/NEt3 at room
temperature affording the butenolide-substituted carbene complexes
[Ru2Cp2(CO)(µ-CO){µ-η1:η3-Cα(H)CβCγ(R)2OC(=O)C(H)] (R = Me, 13a; R = Ph, 13b).
The intermediate cationic [Ru2Cp2(CO)(µ-CO){µ-η1:η3-Cα(H)CβCγ(Me)2OC(OEt)C(H)]+ (12) has been detected in the course of the reaction leading to 13a. Alternatively, the addition of N2CHCO2Et/NHEt2 to 4a gives the 2-furaniminium-carbene
[Ru2Cp2(CO)(µ-CO){µ-η1:η3-Cα(H)CβCγ(Me)2OC(NEt2)C(H)]+ (14). The X-Ray structures of [4a][BPh4], [4c][BPh4], 13a, 13b and [14][BF4] have been determined.
The novel cationic diiron µ-allenyl complexes [Fe2(Cp)2(CO)2(µ-CO){µ-η1:η2α,β
-C(H)=C=C(R)2}][BF4] (R = Me, 7a; R = Ph, 7b) have been obtained in good yields by a
two-step reaction starting from [Fe2Cp2(CO)4]. The solid state structures of [7a][CF3SO3]
has been ascertained by X-Ray diffraction studies. The reaction of 7a with Brönsted bases yields the dimetallacyclopentenone [Fe2Cp2(CO)(µ-CO){µ-η1:η3
-4
C(H)=C(C(Me)CH2)C(=O)}] (16). The reactions of 7a,b with MeCN/Me3NO result in
prevalent decomposition to mononuclear iron species.
The diiron µ-vinyl complex [Fe2Cp2(CO)(µ-CO){µ-µ1:µ2-CH=CH(Ph)}][BF4], 9,
undergoes reduction by means of CoCp2 affording selectively the C−C coupling Fe4
compound [Fe2Cp2(CO)2(µ-CO){µ-µ2-CHCH(Ph)}]2, 16. The cation [9]+ has been
regenerated from 16 in good yield upon treatment with I2. Electrochemical studies have
outlined that the reduction of 9 to 16, occurring at -0.92V, is reversible and proceeds with intermediate formation of the radical species [Fe2Cp2(CO)2
(µ-CO){µ-CHCH(Ph)}], 17. This latter has been characterized by EPR spectroscopy. The structures of 9, 16 and 17 have been optimized for the gas phase by DFT calculations; the computed enthalpy related to the equilibrium 17 16 is ∆H = −12.25 KJ·mol−1. The reaction of 9 with CoCp2 in the presence of excess PhSSPh affords the
mononuclear complex [Fe2Cp2(CO)2(µ-CO){µ-µ2-CHCH(Ph)(SPh)}], 18, in 70% yield.
The new compounds 16 and 18 have been fully characterized by IR and NMR spectroscopy, elemental analysis and X-Ray diffraction studies.
5
2. Introduction
The addition of organic reactants to unsaturated hydrocarbyl fragments, promoted by transition metal species, is a topic arousing great interest, due to the wide applications both in laboratory synthesis and in industrial processes1. In this context, the activation of hydrocarbyl units, bridged coordinated in dinuclear iron or ruthenium complexes bearing ancillary cyclopentadienyl and/or carbonyl ligands, has been widely investigated2. Indeed, dinuclear metal species often provide unconventional reactivity patterns to bridging ligands, as consequence of the cooperativity effects due to the two metal centres working in concert3. Therefore, metal-bound hydrocarbyl groups may be converted into unusual organic species, which could not be attainable through common organic procedures. Furthermore, the bimetallic core may play a key role in the stabilization of the new species, offering the possibility of different coordination fashions.
These reactions, particularly those leading to carbon-carbon bond formation, have attracted interest because they may act as models for heterogeneously catalyzed processes occurring on metal surfaces4.
As a noticeable example, several papers have appeared in the last decade on the chemistry of diiron complexes containing a bridging vinylimminium ligand, [−C(R)=C(R′)C=N(Me)(R′′)]+, held by the frame [Fe2Cp2(CO)2]. These metal
compounds have revealed to be convenient starting materials for the preparation of unusual “organic architectures” in mild conditions, by stepwise functionalization of the C3 chain5. Interesting examples of the potentiality of these materials are the following: i)
6
synthesis of a selenophene-functionalized carbene complex5a; ii) synthesis of ferrocenes substituted at one Cp ring only, via alkyne cycloaddition to the vinyliminium (Scheme 1). Fe Fe C O Cp Cp CO N CH3 H3C Fe R'CCH R R' N CH3 CH3
Scheme 1: Formation of a tetrasubstituted ferrocenyl complex from a bridging vynilimminium dinuclear complex.
7 2.1 Allenyl ligand: types of coordination
Among the unsaturated hydrocarbyl species, allenyl ligands [−C(H)=C=CRR′] in mono-, di- and poly-nuclear complexes have attracted increased attention, since their coupling reactions with unsaturated organics may provide interesting derivatives6. In particular, di- and poly-nuclear species have attracted considerable attention: the unsaturated C3 chain, similarly to the vinylimminium one, readily undergoes coupling
reactions with organic fragments, providing an easy route for the synthesis of new multisite-bound hydrocarbyl ligands.
Allenyls possess diverse bonding capabilities, and each can provide host metal centre(s) with up to 5 electrons through σ and π interactions upon further complexation.
2.1.1 Mononuclear complexes
In mono-nuclear complexes three modes are possible, depending on the number of carbons involved (Scheme 2):
[M] Cα R1 Cβ Cγ R2 R3 [M] Cα R1 Cβ Cγ R2 R3 [M] Cα R1 Cβ Cγ R2 R3 A B C
8
The simple η1 coordination, A, involves only the Cα of the ligand, which is σ−bound
to the metal. A noticeable example7 is depicted in Scheme 3; As in many cases, η1 -allenyl complexes exhibit slow tautomerization to yield an equilibrium mixture with the η1-propargyl isomer. Pt C X Ph3P Ph3P H C C H Ph Pt X Ph3P C Ph3P C C Ph H H
Scheme 3: Reversible interconversion between η1-allenyl and η1-propargyl platinum complexes.
Mononuclear η2-allenyl or metallamethylenecyclopropene complexes represent a small group of compounds in which the (neutral) organic ligand behaves as a 3-electron donor. The η2 coordination, B, involves two metal-carbon interactions: π-interaction with Cα and σ-interaction with Cβ. Two structures8,9 are reported as an
example (Scheme 4). Mo (OMe)3P P(MeO)3 C C C Ph H Ph W P P CO C C S Ph C H Ph S NR2
Scheme 4: Molybdenum (a) and tungsten (b) η2-allenyl complexes.
The η3 coordination mode, C, is the most common one found in mononuclear complexes, as is the case of [Os(PMe3)4(η3-C(Ph)=C=C(H)(Ph)]PF610 and
9
[Pt(PPh3)2(η3-C(Ph)=C=CH2)]PF6 (see Scheme 5).11 This type of coordination consists
of three metal-carbon interactions: σ (through Cα), and π (through the Cα=Cβ double
bond). Pt PPh3 Ph3P C C C Ph H H Scheme 5: η3
-allenyl platinum complex.
2.1.2 Dinuclear complexes
Dinuclear complexes, as well as mononuclear ones, exhibit three possible coordination fashions, which are represented in
Scheme 6. M M Cα Cβ Cγ R R2 R1 M M Cα R Cβ C γ R1 R2 M M Cα R Cβ Cγ R1 R2 A B C
10
A η3-allenyl ligand can become a 5-electron η2:η3 donor upon coordination to two metal centres, as showed for A. Dinuclear compounds containing such η2:η3-allenyl ligand have been known for over a decade, and here one example is reported12.
(OC)2CpW Fe(CO)3 Cα Cβ Cγ Ph H H Scheme 7: µ−η2 :η3
-allenyl iron-tungsten complex.
The only case of type B coordination mode complex is Ru2(CO)6(µ-PPh2)(µ−η1:η2
-C(Ph)=C=CH2), reported by Carty and coworkers13 in 1986. This compound, as well as
its Os2 analogue, show unusual µ−η1:η2β,γ-allenyl coordination14 (Scheme 8). In fact,
allenyls in M2(CO)6(µ-PPh2)(µ−η1:η2-C(R)=C=CR’2) complexes behave as 3-electon
donors binding the metal atoms through the Cα=Cβ bond; 15
(OC)3Ru Ru(CO)3 Cα Ph Cβ Cγ H H P Ph2 Scheme 8 : µ−η1 :η2 β,γ-diruthenium complex.
11
The solid state structures of several µ−η1:η2α,β-allenyl complexes, C, have been
elucidated by X-ray diffraction1,2,16. Thus, the allenyl fragment is differently bent, with the C−C−C angle ranging between 143 and 157°. The Cα=Cβ bond distance (1.36÷1.40
Å) is somewhat lengthened compared to the free C=C double bond, whereas the Cβ=Cγ
bond distance (1.31÷1.35 Å) resembles that of an unperturbed C=C double bond. For homometallic complexes, the Mβ−Cα and Mβ−Cβ bonds are longer than the Mα−Cα
bond, as expected for π and σ interactions respectively (see Scheme 9).15b
Pt Ru Cα Ph Cβ Cγ H H Ph3P CO Ph3P Cp Scheme 9: µ−η1 :η2 α,β -platinum-ruthenium complex. 2.1.3 Trinuclear complexes
In the case of trinuclear allenyl compounds, only one type of coordination has been reported in the literature. These complexes contain an allenyl ligand that is η1−bonded to one metal centre through Cα and η2−bonded to the remaining metal atoms through
Cα=Cβ and Cβ=Cγ. Thus the allenyl behaves as a overall 5-electron donor (see Scheme
12 (OC)3Fe Fe(CO)3 W Cp(CO)2 Cα Cβ Cγ H H Ph (OC)3Ru Pt(CO)(PPh3) Ru (CO)3 Cα Cβ Cγ H H H A B
Scheme 10: Examples of trinuclear allenyl complexes.
2.1.4 Polynuclear complexes
The anionic pentanuclear cluster [Rh2Fe3(CO)10(µ2−CO)3(µ4−η1:η2:η2:η2−
C(Me)=C=CH2)]− 17, whose simplified structure is reported in Scheme 11, represents
the unique case of allenyl metal compound with nuclearity higher than 3. As observed for trinuclear clusters, the allenyl ligand is η1−coordinated trough Cα to one metal and
η2−coordinated trough Cβ=Cγ to the diagonally opposite metal; the Cα=Cβ double bond
is coordinated also to both the remaining metals.
Rh Fe Fe Fe Rh Cα Cβ Cγ H H Me
-Scheme 11: pentanuclear allenyl compound, [Rh2Fe3(CO)10(µ2−CO)3(µ4−η1:η2:η2:η2−C(Me)=C=CH2)]
−
, (carbonyl ligands are omitted for sake of clearness).
13 2.2 Synthesis of dinuclear µµµµ−−−−allenyl complexes
Four synthetic procedures have been reported for the preparation of dinuclear bridging allenyl complexes, and they are described in the following.
2.2.1 Reaction of mononuclear metal η1-allenyls or η1-propargyls with low-valent metal complexes
This method15a,1816a is useful for the synthesis of heterometallic µ−η1:η2α,β-allenyl
complexes. The unattached carbon-carbon multiple bond(s) of the reacting allenyl or propargyl complex is (are) employed in coordination to another metal, thus initiating a sequence of steps leading to a higher nuclearity product (example in Scheme 12).
Cp(CO)2Ru(CH2C=CPh) + Pt(PPh3)2(C2H4) (Ph3P)2Pt C C C Ph H H Ru(CO)Cp
Scheme 12: Synthesis of [Pt(PPh3)2Ru(CO)(Cp )(µ−η1:η2-PhC=C=CH2)]
2.2.2 Reaction of dinuclear metal acetylides with diazomethane
This method was developed by Carty13,15b and coworkers and has been applied to homobimetallic complexes. Reaction of dinuclear metal acetylides with diazomethane results in nucleophilic attack occurring regiospecifically at Cα, to afford a bridging
14 (OC)3Ru Ru(CO)3 P Ph2 C C Ph (OC)3Ru Ru(CO)3 P Ph2 C C C Ph Ph Ph Ph2CN2
Scheme 13: Synthesis of [Ru2(CO)6(µ-PPh2)(µ−η1:η2-PhC=C=CPh2)].
2.2.3 Reaction of binuclear metal carbonyl anions with propargyl halides
This procedure consists of nuclephilic attack of metal carbonyl anions on propargyl cations. For example19, the generic complex [Fe2(CO)6(µ-SR)(µ−CO)]− reacts with a
variety of propargyl halides affording the corresponding dinuclear bridging allenyl complex [Fe2(CO)6(µ-SR)(µ−η1:η2-RC=C=CH2)] (Scheme 14).
(OC)3Fe Fe(CO)3 S R O C (OC)3Fe Fe(CO)3 S R C C C R H H RCCCH2Cl - HCl
Scheme 14: Synthesis of [Fe2(CO)6(µ-SR)(µ−η1:η2-RC=C=CH2)].
2.2.4 Alkyne exchange and Dehydration
Knox and coworkers discovered that a convenient route to µ−η1:η2α,β-allenyl
15
(M1=M2=Fe; M1=Fe, M2=Ru; M1=M2=Ru) to afford the dimetallacyclopentenone [M1M2(CO)(µ−CO)2{µ−η1:η3−C(R1)C(R2)C(O)}(Cp)2] (M1=M2=Ru, 1; M1=M2=Fe, 2;
M1=Fe, M2=Ru, 3) in good yield20 (Scheme 15).
M1 M2 C O O C Cp Cp CO OC R1 CCR2 UV M1 M2 C O Cp Cp OC C C C O R2 R1
Scheme 15: Photolytic insertion of an alkyne into the M−CO bond of [M1M2(CO)2(µ−CO)2(Cp)2] (M
1
=M2=Ru, 1; M1=M2=Fe, 2; M1=Fe, M2=Ru, 3).
While in the case of the mixed iron-ruthenium dimetallacyclopentenone2d a large variety of alkynes undergo insertion in the Ru-CO bond of [RuFe(CO)2(µ−CO)2(Cp)2],
the same reaction involving the diruthenium complex [Ru2(CO)2(µ−CO)2(Cp)2] is
limited to diphenylacetylene20a, affording 1. However, 1 gives alkyne exchange when heated at reflux in toluene with an excess of R1C≡CR2 21; it is important to notice that this reaction may produce an inseparable mixture of two regioisomers, with either R1 or R2 being the substituent at Cα (Scheme 16).
Ru Ru C O Cp Cp OC Cα Cβ C O R1 R2 Ru Ru C O Cp Cp OC Cα Cβ C O R2 R1 Ru Ru C O Cp Cp OC Cα Cβ C O Ph Ph
+
R1CCR2 toluene, reflux16
When the exchange process illustrated in Scheme 16 is carried out by using a terminal alkyne, the reaction is governed by steric factors so that the alkyne-hydrogen will be found at Cα. On the other hand, the reaction is controlled by electronic factors in case of
use of internal alkynes, with the most withdrawing substituent binding to the relatively electron-rich Cβ.
The facile exchange reaction makes complex 1 as a source of the highly reactive intermediate “[Ru2(CO)3(Cp)2]”. By this approach22, it has been possible to prepare the
cationic µ−allenyl complexes [M1M2Cp2(CO)2(µ-CO) {µ−η1:η2−C(H)=C=C(Me)(R)}]
[BF4] (M1=M2=Ru: R=Me, 4a; R=Ph, 4b; M1=Ru, M2=Fe: R=Me, 5 ) in good yields;
the synthesis includes a protonation followed by dehydration step starting from dimetallacyclopentenone species obtained by insertion of an alkynol of general formula HC≡C−C(R1)(R2)OH (Scheme 17). M1 M2 C O Cp Cp OC C C C O C H M1 M2 C O Cp Cp OC C C H HO R2 R1 C R1 R2 CO [BF4] HBF4 - H2O
Scheme 17: Dehydration step affording [M1M2Cp2(CO)2(µ-CO)
17
2.3 Chemistry of dinuclear µµµ−µ−−−allenyl complexes
2.3.1 Fluxionality
NMR studies on the µ−η1:η2−allenyl complex [Fe2(CO)6(µ-SR)(µ−η1:η2
-HC=C=CH2)] carried out by Seyferth and coworkers showed that the methylene protons
equivalent at room temperature on the NMR timescale19. It was determined that there was a close structural similarity between the allenyl bonding mode and that of a bridging σ−π vinyl, both of which undergoing a fluxional process known as “windshield wiper” exchange23. This process consists of rapid exchange of the σ and π bonds of the bridging allenyl between the two metal centres. Analogous evidence has been found for the diruthenium cationic µ−allenyl complex 4a (Scheme 18) but not for 4b. This has been attributed to the presence of a phenyl substituent on Cγ in the latter.
The phenyl group, being sterically bulkier than the methyl, prefers axial orientation with respect to the Ru-Ru axe; a σ/π exchange would lead the phenyl substituent in a sterically unfavoured position21.
Ru Ru C O Cp Cp OC C C H C Mea Meb CO Ru Ru C O Cp Cp OC C + C H C Mea Meb CO Ru Ru C O Cp Cp OC C C H C Meb Mea CO -+ + +
18 2.3.2 Reactivity with nucleophilic reagents
In principle, the µ−allenyl ligand contains three sites susceptible of nucleophilic attack, i.e. the α, β and γ carbons. Actually, to date examples are dominated by the attack at the β carbon.
For example, Carty and coworkers24 investigated the reaction of the diruthenium µ−η1:η2−allenyl complex [Ru2(CO)6(µ-PPh2)(µ−η1:η2-PhC=C=CH2)] with carbon,
phosphorus and nitrogen nucleophiles. All the latter attacked regiospecifically at Cβ to
afford novel, zwitterionic five-membered dimetallacycles (Scheme 19).
(OC)3Ru Ru(CO)3 C Ph C C H H P Ph2 (OC)3Ru Ru(CO)3 C Ph C C H H P Ph2 NHR + -(OC)3Ru Ru(CO)3 C Ph C C H H P Ph2 PPh2R + -(OC)3Ru Ru(CO)3 C Ph C C H H P Ph2 C -NtBu + RNH2 PPh2R t BuNC H
19
When a bidentate nucleophile is employed, the attack may occur at the metal centre, as it was observed in the reaction of [Ru2(CO)6(µ-PPh2)(µ−η
1
:η2-PhC=C=CPh2)] with
dppe and dppm25. The aptitude of dppe to behave as a bidentate ligand towards a single metal centre, makes the allenyl displacing from the η2-coordination. In the case of dppm, the latter bridges the two metals to form a five-membered cycle without affecting the Ru-allenyl binding (Scheme 20).
(OC)3Ru Ru(CO)3 C Ph C C Ph Ph P Ph2 (OC)3Ru Ru C Ph C C H H P Ph2 (OC)3Ru Ru(CO)2 C Ph C C H H P Ph2 CO CO Ph2 P P Ph2 Ph2P C H2 PPh2 dppe dppm - CO
Scheme 20: Reaction of [Ru2(CO)6(µ-PPh2)(µ−η 1
20
Further example26 of nucleophilic attack occurring at the Cβ is given by the reaction of
[(PPh3)2Pt(µ-H)(µ−η1:η2-C(R)=C=CH2)Ru(CO)Cp] with water, promoted by acidic
alumina, affording the hydrido-alkylidene complexes [(PPh3)2Pt(µ-H)(µ−η1:η1
-C(R)C(O)CH3)Ru(CO)Cp] (R = H, Ph) (Scheme 21).
(Ph3P)2Pt C C C R H H Ru(CO)Cp (Ph3P)2Pt C C H3C R Ru(CO)Cp O H2O
Scheme 21: Reaction of [(PPh3)2Pt(µ-H)(µ−η1:η2-C(R)=C=CH2)Ru(CO)Cp] with water.
The reactions of homonuclear metal hexacarbonyl µ−allenyl complexes have received considerable attention from Doherty and coworkers. They found that several diiron and diruthenium compounds of general formula [M2(CO)6(µ-X)(µ−η1:η2-C(R)=C=CR12)]
(X = PPh2, SBu-t; R = H, Ph; R12 = H2, Ph2) were able to react readily with primary
amines27, alcohols28, organolithium reagents16b, isocyanides29, phosphite esters6j, and monodentate and bidentate phosphines6i,6j,25,30. In most cases, the nucleophile added to the allenyl ligand, although additions to the metal and the carbonyl ligand have been described too. Examples of nucleophilic attack respectively at the allenyl Cβ27b (Scheme
22, I) and at CO with consequent intramolecular rearrangement 28 (Scheme 22, II) are presented.
21 (OC)3Fe Fe(CO)3 Cα H Cβ Cγ H H P Ph2 (OC)3Fe Fe(CO)3 Cα Cβ Cγ P Ph2 (OC)3Fe Fe(CO)3 Cα Cβ Cγ H H P Ph2 H H H H N H Cy + (I) (II) H D O MeO CyNH2 MeOD
Scheme 22: Reaction of [Fe2(CO)6(µ-PPh2)(µ−η1:η2-C(H)=C=CH2)] with CyNH2 (I) and
MeOD (II).
Preliminary studies on the chemistry of the diruthenium µ−allenyl complex 4a have shown that the allenyl ligand is reactive towards neutral organic molecules such as alkynes and diazocompounds21. Moreover, compounds 4a and 4c react readily with MeCN/Me3NO to give the acetonitrile adducts [Ru2Cp2(CO)(MeCN)(µ-CO)
22 Ru Ru C O Cp Cp OC C C H C R Me CO + Ru Ru C O Cp Cp OC C C H C R Me NCMe + CH3NO/MeCN - CO2
Scheme 23: Reaction of [Ru2Cp2(CO)2(µ-CO) {µ−η1:η2−C(H)=C=C(Me)(R)}][BF4] with
Me3CN/Me3NO.
The chemistry of the derivatives 6 offers promise in that acetonitrile often acts as a labile ligand in transition metal chemistry. For dinuclear species, acetonitrile removal is a crucial step in allowing coordination, at one of the two metal centres, of either anionic nucleophiles such as hydride, halides and pseudo-halides31, or neutral organic fragments2g,2i,4a,4b,5e.
The substitution of a carbonyl ligand by an acetonitrile molecule in cationic diruthenium allenyl complexes allows the introduction of hydride and halide ions to the metal sites under mild conditions22. The resulting products undergo successive rearrangements involving the bridging allenyl ligand promoted by thermal treatment or filtration through alumina. Hydride migration from the metal site to the Cα carbon of the
allenyl moiety provides a µ−η1:η2−allene product which is further convertible into a more stable vinyl-alkylidene derivative, by hydrogen migration from Cα to Cβ (Scheme
23 Ru Ru C O Cp Cp OC Cα Cβ H Cγ Ph Me NCMe + Ru Ru H Cp Cp OC Cα Cβ H Cγ Ph Me CO Ru Ru Cp Cp OC Cα C β H Cγ Ph Me CO H Ru Ru C O Cp Cp OC Cα Cβ H Cγ Ph Me H NaBH4 - MeCN
Scheme 24: Reaction of 6b with NaBH4.
2.3.3 Reactivity with electrophilic reagents
Heterometallic µ−allenyl complexes may react with the electrophilic p−TolSO2NCO
(TSI) to generate [3+2] cycloaddition products6g, as illustrated in Scheme 25.
(CNt-Bu)(Ph3P)Pt C C C Ph H H Ru(CO)Cp (CNt-Bu)(Ph3P)Pt C C Ru(CO)Cp Ph H2C C N Tos O TSI
24
Other electrophilic alkenes, e.g. ClSO2NCO, TCNE and fumaronitrile, react with
µ−allenyl complexes by either Mα−Mβ bond cleavage or uncharacterized
decomposition6g. However, it has been reported that [Au(PPh3)2]+ adds to the Cβ carbon
of the allenyl ligand in [(PPh3)2Pt(µ−η1:η2-C(R)=C=CH2)Ru(CO)Cp] (R = H, Ph) to
generate a η3−allyl complex32.
2.3.4 Other reactions
Bridging allenyl complexes sometimes display unusual propensity to fragmentation into mononuclear metal compounds upon treatment with CO16a (Scheme 26).
(Ph3P)2Pt C C C H H H M(CO)Cp Cp(OC)2M C C C H H H Pt(PPh3)2(CO)2 + CO
Scheme 26: Fragmentation of [(PPh3)2Pt(µ−η1:η2-C(H)=C=CH2)Ru(CO)Cp].
The observed fragmentation may be favoured by relatively weak Pt−M bond. Some of these reactions are reversed under Ar atmosphere.
25 2.4 Vinyl ligand: types of coordination
Metal vinyl complexes have attracted attention because of their significance in organometallic synthesis and catalysis33.
One area of great interest, in the light of the proposition that µ−vinyl groups are involved in the initiation and propagation steps of the Fischer-Tropsch synthesis34, are the C−C bond forming reactions of the µ−vinyl ligand with small organic molecules. Due to the presence of a C−C double bond, the vinyl ligands exhibit different coordination modes in mono-, di- and polynuclear transition metal complexes.
2.4.1 Mononuclear complexes
The organic vinyl fragment, [CR=CR2]−, can coordinate through one or both sp2
carbons. In the first case, the vinyl ligand acts as a simple σ donor, as highlighted in Scheme 27.
R1
R2 R3
LnM
Scheme 27: η1−vinyl / σ− vinyl.
Much more attractive is the second case, where the coordination of both carbons increases the electron count at the metal(s) by two electrons with respect to the situation of simple σ−bound vinyl ligand (Scheme 28). The discovery that the vinyl ligand could adopt this type of coordination led to the speculation that η2−vinyl complexes could be
26
involved along catalytic pathways involving coordinatively unsaturated alkenyl species35. LnM R1 R2 R3 LnM R1 R2 R3 η2−vinyl 1-metallacyclopropene
Scheme 28: Nomenclature and Labelling for LnM(η2−C(R)=CR2).
Two prominent geometrical features define the η2−vinyl ligands. First, the ligand is bound asymmetrically to the metal(s) through both carbons. Second, the vinyl fragment is not planar. Conversely, the CR2 unit is rotated so that the carbon-carbon double bond
characteristic of the vinyl unit is compromised in favor of double bond character between Cα and the metal. The net result is that the substituents on the β−carbon are
approximately orthogonal to the MC2 plane and the distinction between cis and trans
sites associated with vinyl fragments is lost.
The [η2−C(R)=CR2)]− ligand has been described as a η2−vinyl ligand or as a
η2−alkenyl ligand, and the resulting product has been termed either a η2−vinyl complex or a 1-metallacyclopropene complex (Scheme 28). However, the η2−vinyl term has enjoyed widespread use in the literature, and the origin of the fragment and the reactivity patterns that yield and consume these ligands are intrinsically associated with the vinyl name. On the other hand, Casey has argued for adoption of the
1-27
metallacyclopropene nomenclature36. Indeed, the 1-metallacyclopropene structural representation is congruent with the ground state structures and spectroscopic properties of these complexes.
It is important to remember that there are two components to the total metal-(C=C) bonding: (a) overlap of the π-electron density of the C=C bond with a σ-type acceptor orbital on the metal atom and (b) a “back bond” resulting from flow of electron density from the filled metal dxz or other dπ−pπ hybrid orbitals to antibonding orbitals of the
carbon atoms. Of course, the donation of π-bonding electrons to the metal σ orbital and the introduction of the electrons into the π-antibonding orbital both weaken the π bonding in the C=C bond, so that the carbon atoms bound to the metal approach tetrahedral hybridization. Thus it is possible to formulate the bonding as involving two normal 2c-2e metal−carbon bonds in a metallacycle.
Thus, in vinyl complexes the preference of a η2−coordination rather that a η1− is driven by the electronic character of the ligands present in the coordination sphere. Strongly donating substituents force the vinyl ligand to adopt a η1 (2e−) binding mode rather than a η2 (4e−) binding mode, which is preferred when more withdrawing substituents are present, as highlighted by the examples37 reported in Scheme 29.
28 W CF3 R3P CF3 Cp Cl F3C F3C W Me H Ph Cp Cl Ph Me Ι II
Scheme 29: Examples of η1−vinyl (I) and η2−vinyl (II) complexes of tungsten.
2.4.2 Dinuclear complexes
In the case of dinuclear complexes, only one coordination fashion is reported for the bridge vinyl ligand. The bonding in µ−vinyl complexes has been described in terms of contribution from three Valence Bond structures (Scheme 30).
M1 M2 Cα R1 Cβ R2 R3 M1 M2 Cα R1 Cβ R2 R3 M1 M2 Cα R1 Cβ R2 R3 A B C
Scheme 30: VB structures of the bonding in µ−vinyl complexes.
Cationic µ−vinyl complexes undergo nucleophilic attack, including hydride addition, at the β carbon to generate µ−alkylidene complexes44a,45b,48.
In most cases the metal−metal bond is preserved in the structure of dinuclear µ−vinyl complexes (example38 in Scheme 31, A), however Werner and coworkers39 reported the
29
structure of dinuclear doubly vinyl-bridged iridium complexes where the two metal centres are not bonded to each other (Scheme 31, B).
(OC)4Fe Co(CO)3 H3C CH3 CH3 Ir Ir H H H H H H A B
Scheme 31: Examples of metal-metal bonded (A) and not bonded µ−vinyl dinuclear complexes (B).
2.4.3 Polynuclear complexes
Examples of trinuclear vinyl complexes are quite common; in these compounds the coordination mode of the vinyl ligand is analogous to the one adopted in dinuclear complexes, as the maximum number of carbons which can be involved in the metal−carbon bonds is two. An example40 of a trinuclear µ−vinyl complex is reported in Scheme 32. These compounds adopt the µ−η2−vinyl structure (Scheme 32, A) with the trans configuration. The related cis−PhC=CHPh complex and the furyl complex are similar, except that they adopt the alternative configuration reported in Scheme 32, B, in which a clash of the 1-substituent with an axial CO at the Os(CO)4 unit is avoided.
30 (OC)4Os Os(CO)3 Os (CO)3H R H H (OC)4Os Os(CO)3 Os (CO)3H R H H A B
Scheme 32: Structures of diosmium complexes of general formula [Os3(CO)10(µ−H)(µ−C(H)=CHR)].
To my knowledge, the only example of a vinyl-containing cluster with a nuclearity higher than 3 (three) is the tetranuclear anionic complex [Ir4(CO)11(µ−C(Ph)=C(H)(Ph)]− 41. In this compound, the metallic framework is
tetrahedral with the vinyl ligand being η1-bound and pointing away from the core (Scheme 33). Ir Ir Ir CO CO OC OC OC CO Ph H H OC Ir OC CO OC
31 2.5 Synthesis of dinuclear µ−µ−µ−µ−vinyl complexes
There are numerous methods available for the preparation of bimetallic µ−vinyl complexes: these include insertion of a metal into vinylic C−S42 or C−halogen43 bond, rearrangement of µ−alkylidyne complexes and their reaction with alkenes44, β−hydride abstraction from µ−alkylidene complexes45, addition of alkynes to transition metal carbonyl hydrides46, and protonation of alkyne-bridged bimetallic complexes47 and dimetallacyclopentenones48.
2.5.1 Insertion of a metal into vinylic C−S or C−halogen bond
In 1961 King and coworkers42 discovered that the reaction of triiron dodecacarbonyl with a vinyl sulfide resulted in insertion of two iron centres into the vinylic C–S bond of the sulfide, affording the vinyl diiron complexes [Fe2(µ−SR)(CO)6(µ−CH=CH2)] (R =
Me, Et, CHCH2, iPr) (Scheme 34: Reaction of a). By a similar method, a vinyl diiron
compound may be synthesized via insertion of the two iron centres of Fe2(CO)9 in the
C–halogen bond of a haloalkene43(Scheme 34b).
(OC)3Fe Fe(CO)3 X H H H C C H H X H Fem(CO)n +
Scheme 34: Reaction of iron carbonyls with (a) vinyl sulfides (m=3, n=12, X=RS) and (b) haloalkenes (m=2, n=9, X = F, Br, I).
32 2.5.2 Rearrangement of µ−alkylidyne complexes
A convenient way to prepare µ−vinyl derivatives involves rearrangement of alkylidyne diiron complexes, such as [Fe2Cp2(CO)2(µ−CO)(µ−C(CH2)3)CH3)][PF6]44, upon
heating. The preparation of the alkylidyne is easily achieved by reacting the methylidyne complex [Fe2Cp2(CO)2(µ−CO)(µ−CH))][PF6] with the appropriate alkene.
This hydrocarbation reaction proceeds by a regioselective addition of the µ−C−H bond across the C=C bond49 (Scheme 35).
Fe Fe CO OC Cp Cp C O C H + Fe Fe CO OC Cp Cp C O C + Fe Fe CO OC Cp Cp C O H H + CH2=CHCH2CH3 88°C
Scheme 35: Reaction of diiron methylidyne complex with 1-butene and rearrangement of the alkylidyne product affording the corresponding µ−vinyl species.
The conversion of µ−alkylidene complexes to µ−vinyl ones proceeds with net migration of a hydrogen atom from Cβ to Cα. Studies carried out with deuterated
derivatives showed that the migration involved the β−carbon hydrogens49.
2.5.3 β−Hydride abstraction from µ−alkylidene complexes
β−Hydride abstraction from mononuclear metal alkyls, as well as the reverse reaction, are well known processes in organometallic chemistry50. This type of reaction may be used conveniently to prepare vinyl dinuclear complexes. For example45b, when the
33
µ−alkylidine [Fe2Cp2(CO)2(µ−CO)(µ−CHCHR2)] are treated with the trityl cation
Ph3C+, abstraction of a hydride from Cβ takes place, affording the corresponding
cationic vinyl complex, as highlighted in Scheme 36.
Fe Fe CO OC Cp Cp C O C Fe Fe CO OC Cp Cp C O H R R + CHR2 H Ph3C+
Scheme 36: Reaction of [Fe2Cp2(CO)2(µ−CO)(µ−CHCHR2)] with Ph3C +
.
2.5.4 Addition of alkynes to transition metal carbonyl hydrides
The reaction of metal hydrides with alkynes provides a general route to µ−vinyl ligands. An equimolar mixture of Fe2(CO)9, R1C≡CR2 and [HFe(CO)4]− in THF affords
the µ−vinyl compounds of general formula [Fe2(CO)6(µ−CO)(µ−CR1=CHR2)]− 46
(Scheme 37). (OC)3Fe Fe(CO)3 C C C O R1 R2 H Fe2(CO)9 + HFe(CO)4- R 1 CCR2
Scheme 37: Synthesis of [Fe2(CO)6(µ−CO)(µ−CR 1
34
The synthesis seems to occur with alkyne activation by Fe2(CO)9 followed by
interaction with the metal hydride.
2.5.5 Protonation of alkyne-bridged bimetallic complexes and
dimetallacyclopentenones
Insertion of alkynes into dinuclear species affords two kind of products, i.e. alkyne-bridged bimetallic complexes and dimetallacyclopentenones, which can be further protonated to obtain µ−vinyl dinuclear complexes.
For example, the protonation of the µ−alkyne dimolibdenum [Mo2Cp2(CO)3(µ−CO)(µ−CHCH)] yields the corresponding µ−vinyl derivative
[Mo2Cp2(CO)3(µ−CO)(µ−CHCH2)] 47a (see Scheme 38).
Mo Mo CO OC Cp Cp C O Mo Mo CO OC Cp Cp C O C C H H H + CO C C H H CO H+
Scheme 38: Protonation of the µ−alkyne dimolibdenum complex. [Mo2Cp2(CO)3(µ−CO)(µ−CHCH)]
In a similar way, the dimetallacyclopentenone species [M2Cp2(CO)2(µ−CO)
(µ−η1:η3−CR1=CR2C(O))] (M=Fe, Ru)48, synthesized by photolytic insertion of alkynes into the M−CO bond (see paragraph 2.2.4), may be easily protonated upon treatment with HBF4, resulting in immediate cleavage of the alkyne–CO bond. The “alkyne”
35
portion of the dimetallacycle is protonated to give the µ-vinyl complexes [M2Cp2(CO)2(µ-CO)(µ-CR1=CHR2)][BF4] (M = Fe, Ru), in which R1 and R2 are in
relative cis position; as a result of the protonation, the acyl group is converted into a terminal carbonyl ligand.
M C C C M Cp Cp CO OC C O R1 R2 O M M Cp Cp CO OC C O R1 R2 H HBF4 [BF4]
Scheme 39: Protonation of the dimetallacyclopentenone species [M2Cp2(CO)2(µ−CO)(µ−η
1
:η3−CR=CRCO)] (M=Fe, Ru).
There is strong evidence that, whether preceded by metal protonation or not, the acyl moiety in the dimetallacyclopentenone undergoes protonation prior to the production of the µ−CR1=CHR2 unit. It is therefore apparent that the reaction proceeds with proton transfer from the {C=O} group to the {C(R2)} carbon, with simultaneous C−C bond cleavage.
36 2.6 Chemistry of dinuclear µµµ-vinyl complexes µ
2.6.1 Fluxionality
As well as already highlighted for µ−η1:η2−allenyl complexes, also the µ−vinyl ones exhibit fluxionality. In the 1H NMR spectra of [Fe2Cp2(CO)4(µ-CO)(µ−η1:η2
-C(H)=CHR)] complexes, the non-equivalent cyclopentadienyl groups give rise to two resonances. Upon warming, the two peaks coalesce to a single averaged cyclopentadienyl resonance as first showed by Knox48. The fluxional process that exchanges the environment of the cyclopentadienyl groups involves movement of the β−carbon from one iron centre to the other. During this process, Cα is always bonded to
both iron centres while the Cβ is bonded to only a single iron. A convenient way of
describing the µ−vinyl system is in terms of the 1,2-diiron bicyclopropane structure (Scheme 40). Fe Fe R H H Fe Fe H H R
Scheme 40: Fluxionality of µ−vinyl diiron cation.
2.6.2 Reactivity with nucleophilic reagents
The reactivity of µ−vinyl dinuclear complexes with nucleophiles may involve attack at the α− or β−carbon of the vinyl ligand, or at one of the two metal centres. The
37
reactivity of the previously described µ−vinyl complexes of general formula [M2Cp2(CO)2(µ−CO)(µ−CR1=CHR2)][BF4] (M=Fe, Ru) offers an interesting example
of the chemical behavior of these systems in the presence of nucleophiles.
Both diruthenium and diiron µ−vinyl complexes [M2Cp2(CO)2(µ−CO)
(µ−CR1=CHR2)][BF4] are attacked by nucleophiles at the β−carbon to yield the
corresponding µ−carbene complexes. This reaction completes a sequence of transformation (see Scheme 15 and Scheme 39) by which an alkyne is converted to a carbene coordinated at a dinuclear metal centre. Interest in such complexes has been due to the fact that they may serve as models for metal surface-bound carbenes, which act as intermediate species in the Fischer-Tropsch synthesis of hydrocarbons51. For example, treatment of the µ−vinyl [M2Cp2(CO)2(µ−CO)(µ−CR1=CHR2)][BF4] (M=Fe, Ru) with
NaBH448a determines rapid hydride addition to the β−carbon, generating the appropriate
µ−carbene complex (Scheme 41).
M M Cp Cp CO OC C O Cα Cβ R1 R2 H M M Cp Cp CO OC C O Cα Cβ R1 R2 H NaBH4 H +
Scheme 41: Reaction of [M2Cp2(CO)2(µ−CO)(µ−CR1=CHR2)][BF4] (M=Fe, Ru) with NaBH4.
As previously described in paragraph 2.3.2 (Scheme 23) for analogous diruthenium µ−allenyl complexes, also the diruthenium µ−vinyl compounds show substitution of
38
one terminal CO with MeCN at one of the metal centres in the presence of Me3NO52
(Scheme 42). Ru Ru Cp Cp CO OC C O Cα Cβ R1 R2 H Ru Ru Cp Cp NCMe OC C O Cα Cβ R1 R2 H CH3NO/MeCN - CO
Scheme 42: Reaction of [Ru2Cp2(CO)2(µ−CO)(µ−CR1=CHR2)][BF4] with MeCN/Me3NO.
Knox and coworkers53 (Scheme 43) highlighted how the complex [Ru2Cp2(CO)(MeCN)(µ−CO)(µ−CH=CH2)]+, (I), underwent nucleophilic attack of the
chloride ion at one metal centre, to generate the chloro-derivative (II). Further treatment with LiCu(Me)2 gave substitution of the chloride with a methyl group, resulting in
formation of (III). Although quite stable in the solid state, (III) is unstable in solution due to slow methyl migration first to Cα, (IV), and then to Cβ, (V).
39 Ru Ru Cp Cp NCMe OC C O Cα Cβ H H H Ru Ru Cp Cp Cl OC C O Cα Cβ H H H LiCl + Ru Ru Cp Cp Me OC C O Cα Cβ H H H LiCu(Me)2 Ru Ru Cp Cp CO OC Cα Cβ H H H Me Me migration to Cα Ru Ru Cp Cp CO OC H Cα Cβ H Me H Me migration to Cβ
(I) (II) (III)
(IV) (V)
Scheme 43: Progressive reactivty of [Ru2Cp2(CO)(MeCN)(µ−CO)(µ−CH=CH2)] +
with LiCl and LiCu(Me)2.
2.6.3 Combination with alkenes
The reaction of [Ru2Cp2(CO)(MeCN)(µ−CO)(µ−CH=CH2)]+ with ethylene has been
reported52. In the light of the mechanistic studies, it has been demonstrated that the first step is the displacement of the labile acetonitrile from (I) to give the transient µ-vinyl/ethylene complex (II), as shown in Scheme 44. Once ethylene is coordinated, carbon-carbon bond formation between it and the α(µ)−vinyl carbon occurs rapidly. The process can be viewed as a reductive elimination (2 Ru−C → C−C), which generates the dimetallacycles (III) containing a sixteen-electron ruthenium centre. Then, the latter compound promotes β−elimination of one of the originally ethylenic hydrogen, so to restore the electronic saturation at the dimetal frame and afford (IV). It is noteworthy that the µ−vinyl precursor of (I) is obtained by oxidation of the ethylene
40
complex [Ru2Cp2(CO)3(C2H4)]54, so that the overall scheme represents the sequence of
reactions describing the coupling of two ethylene molecules at a dinuclear metal centre.
Ru Ru Cp Cp NCMe OC C O C C H H H + (I) Ru Ru Cp Cp OC C O C C H H H + (II) H2C CH2 H2C CH2 Ru Ru Cp Cp OC C O + (III) HC C H2 CH2 CH2 Ru Ru Cp Cp OC H + (IV) CO H2C C H H C CH2
Scheme 44:Combination of [Ru2Cp2(CO)(MeCN)(µ−CO)(µ−CH=CH2)]+ with ethylene.
2.6.4 Other Reactions
As stated for the µ−allenyl complexes, also the µ-vinyl ones may display sometimes propensity to fragmentation into mononuclear metal complexes. For example55, treatment of [Fe2Cp2(CO)2(µ−CO)(µ−CH=C(H)(CO2Et))][BF4] with NaI and CO
results in Fe−Fe bond cleavage and formation of [FeCp(CO)2(η1−CH=C(H)(CO2Et))]
41 Fe Fe Cp Cp CO OC C O C C H CO2Et H + Fe OC Cp CO C C H CO2Et H NaI, CO
Scheme 45: Fragmentation of [Fe2Cp2(CO)2(µ−CO)(µ−CH=C(H)(CO2Et))][BF4].
42 2.7 Objective
The object of the present Thesis is the study of the reactivity of diiron or diruthenium complexes containing the [M2Cp2(CO)3] unit and a bridging unsaturated ligand (i.e.
allenyl or vinyl), with the purpose to obtain novel organic fragments by functionalization of the bridging hydrocarbyl ligand, through selective synthetic pathways which may be favored by the dinuclear frame. Moreover, the study might give a contribution to the understanding of mechanistic aspects concerning industrial processes (e.g. Fischer-Tropsch).
On considering that the large majority of the known allenyl complexes are neutral, we have decided to focus our attention on the cationic [M2Cp2(CO)2
(µ-CO){µ-C(H)=C=CRR′}]+ (M = Ru, 4; M = Fe, 7), whose positive charge should basically enhance the reactivity with nucleophiles.
In this Thesis the synthesis of the new diiron µ-allenyl complexes [Fe2(Cp)2(CO)2
(µ-CO){µ-η1:η2α,β-C(H)=C=C(R)2}][BF4] (R = Me, 7a; R = Ph, 7b) will be reported, and
the chemistry of both diiron and diruthenium complexes will be discussed.
Then, we will extend the study to the µ−vinyl complex [Fe2Cp2(CO)2(µ−CO)(µ−CH=CHPh)]+, 9, which is easily prepared from the
dimetallacyclopentenone precursor [Fe2Cp2(CO)2(µ−CO)(µ−C(H)C(Ph)C(O)], 8. A
43
3 Results and Discussion
3.1 Dimetallacyclopentenones: synthesis and characterization
Diiron and diruthenium dimetallacyclopentenone complexes represent convenient precursors for both µ−allenyl and µ−vinyl species, as previously stated in the introduction. Their synthesis is a well-known process, which involves alkyne insertion into the M−CO bond of [M2Cp2(CO)4] (M = Fe, Ru) under UV radiation. Photolytic
insertion of alkynols (Scheme 46a) or alkynes (Scheme 46b) has been performed in order to obtain the desired products.
In the case of complexes 1 and 2, addition of a proton results in dehydration of the inserted alkynol and formation of the allenyl ligand. Instead, in the case of complex 8, proton attack occurs at Cβ, resulting in formation of the vinyl ligand. In both cases the
generation of the new hydrocarbyl moiety is accompanied by the cleavage of the C−C bond and the conversion of the acyl group into a terminal carbonyl ligand.
44 M C C C M Cp Cp CO OC C O H R O Fe Fe Cp Cp CO OC C O C H C Ph H H+ + M M Cp Cp CO OC C O C H C C R1 R1 +
(M=Ru, R=C(Me)2OH, 1a; M=Ru, R=C(Ph)2OH, 1b)
(M=Fe, R=C(Me)2OH, 2a; M=Fe, R=C(Ph)2OH, 2b)
(M = Fe, R=Ph, 8)
(M= Ru, R1 = Me, 4a; M = Ru, R1=Ph, 4b)
(M= Fe, R1 = Me, 7a; M = Ru, R1=Ph, 7b)
Allenyls
R = C(R1)2OH
Vinyls
R = Ph
Scheme 46: Dimetallacyclopentenone species as precursors for allenyl and vinyl complexes.
Compounds 1, 2 and 8 were characterized by IR, NMR and, in the case of 8, by X-ray crystal structure.
Infrared spectra of the dimetallacyclopentenone species display peculiar bands due to the presence of a terminal-, a bridging carbonyl, and an acyl group at 1992-1969 (s), 1790-1805 (s) and 1731-1754 cm-1 (m), respectively. Moreover, a band due to the hydroxyl group appears at ~3300 cm-1 in the spectra obtained on the products of alkynol insertion. The IR spectrum in CH2Cl2 of complex 2b is reported in Figure 1: FT-IR Spectrum (CH2Cl2) of complex 2b in the carbonyl stretching region.
45 2087,8 2000 1950 1900 1850 1800 1750 1700 1655,1 2,2 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 77,9 cm-1 %T 1705 1974 1795
Figure 1: FT-IR Spectrum (CH2Cl2) of complex 2b in the carbonyl stretching region.
Relevant features regarding both 1H-NMR and 13C-NMR spectra are represented by the resonances of the Cα−H unit, which are typical for a µ−carbene (13C-NMR: δ =
152-181 ppm; 1H-NMR: δ = 10-13 ppm). On the other hand, the Cβ−R chemical shifts are
characteristic of a coordinated olefinic carbon (13C-NMR: δ = 13-60 ppm).
In the case of complex 8, crystals suitable for X-ray analyses were collected from a dichloromethane solution layered with Et2O, at –243K. The structure (see Figure 2) is
consistent with that of [Ru2Cp2(µ−CO)(CO)(µ−η1:η3−C(Ph)C(Ph)C(O)] described by
Knox and coworkers20a. The two iron atoms are at a distance (2.561 Å) typical of a single bond and are bridged symmetrically by a carbonyl group. The Fe(1) atom also
1973, CO
1795, µ−CO
46
carries a terminal carbonyl, and each metal atom has a cyclopentadienyl ligand bound in a η5-fashion. The coordination at the iron atoms is completed by a HC=C(Ph)C(=O) species, derived from the coupling of diphenylacetylene and carbon monoxide, bridging the two iron atoms. The C(13)–C(14) bond length (1.443 Å) is within the range for a coordinated double bond, showing evidence of some π character57.
The cyclopentadienyl ligands on the two metal atoms are mutually cis with respect to the metal-metal axis.
Figure 2: View of the structure of complex 8. The H-atoms have been omitted for clarity. Thermal ellipsoids are at the 30% probability level. Only the main images of the disordered Cp
47
3.2 Chemistry of the cationic diruthenium µ−µ−µ−allenyl complexes µ−
3.2.1 Synthesis and characterization
By following the literature procedure (see Introduction), the diruthenium allenyl complex [Ru2(Cp)2(CO)2(µ-CO){µ-η1:η2α,β-C(H)=C=C(Me)2}][BF4]22 (4a) and the
unreported [Ru2(Cp)2(CO)2(µ-CO){µ-η1:η2α,β-C(H)=C=C(Ph)2}][BF4] (4b) have been
synthesized. The diruthenium compounds 4 are known in the form of tetrafluoroborato salts, nevertheless no crystallographic description has been reported heretofore and only limited information have appeared on the reactivity22. Such diruthenium species can be prepared in good yields by a three-step route, see Scheme 47, starting with photolytic insertion of diphenylacetylene into the Ru−CO bond of [Ru2Cp2(CO)4]20, affording the
cyclopentenone [Ru2Cp2(CO)(µ-CO){µ-η1:η3-C(Ph)=C(Ph)C(=O)}]. Successive
alkyne-exchange reaction with alkynol, in thf solution at reflux temperature for 4 hours, results in the formation of [Ru2Cp2(CO)2(µ-CO){µ-η1:η3
-C(H)=C(C(R)(R')(OH))C(=O)}] (R = R' = Me, 1a; R = Ph, R' = Me, 1b). The latter can be isolated by filtration through an alumina column. Further treatment with HBF4 in thf
48 Ru Ru C O O C Cp Cp CO OC UV Ru Ru C O Cp Cp OC C C C O Ph Ph PhCCPh Ru Ru C O Cp Cp OC C C C O C(R)2OH H HCCC(R)2OH thf, reflux HBF4, thf - H2O Ru Ru C O Cp Cp CO OC Cα H Cβ Cγ R R
(R=Me, 4a; R=Ph, 4b) (R=Me, 1a; R=Ph, 1b)
[BF4]
Scheme 47: Preparation of cationic diruthenium µ-allenyl complexes.
The new complexes 4a,b have been fully characterized by IR and NMR spectroscopy, and elemental analysis.
The IR spectra (in CH2Cl2 solution) of 4 display three absorptions ascribable to two
terminal carbonyl ligands and one semi-bridging carbonyl (e.g. for 4a at 2039, 2017 and 1872 cm−1, respectively).
49 2236,2 2200 2100 2000 1950 1900 1850 1800 1750 1716,5 48,6 50 55 60 65 70 75 80 85 90 95 100 101,5 cm-1 %T 2039 2017 1872
Figure 3: FT-IR solid state spectrum of complex 4a in the carbonyl region.
The NMR spectra of 4 at room temperature show broad signals, as result of an exchange process. This process probably consists of σ-π “windshield wiper” motion of the allenyl moiety, in accord with what observed formerly in analogous bridged-allenyl dinuclear complexes16b,6e. Readable 1H-NMR spectra could be recorded at 233K in CD3CN solution. The spectra exhibit a single set of resonances. Relevant feature is
represented by the CαH resonances, falling at typical high-frequency, in accordance with
the alkylidene character [e.g. in the case of 4b: δ(1H) = 10.96 ppm; δ(13C) = 130.9 ppm]. The allenyl carbons Cβ and Cγ are found at ca. 150 and 125 ppm, respectively
The anion-exchange reactions reported in Scheme 48 were used to obtain the crystalline salts [4a][BPh4] and [4c][BPh4] (see Experimental), and their solid-state
structures were solved by X-ray diffraction studies.
2039, CO 2017, CO
50 Ru Ru C O Cp Cp CO OC Cα H Cβ Cγ R R [BF4] Ru Ru C O Cp Cp CO OC Cα H Cβ Cγ R R [BPh4] NaBPh4 thf
Scheme 48: Anion-exchange reaction to obtain the crystalline salts [4a][BPh4] and [4c][BPh4].
The ORTEP representations are shown in Figure 4 and Figure 5. The structures are based on a cis-[Ru2(Cp)2(CO)2(µ-CO)] core, coordinated to the bridging µ-η1:η2
-C(H)=C=C(R)2 allenyl ligand. The bonding parameters of the latter are as expected for
this class of ligands, with bent C(14)–C(15)–C(16) bond [154.8(10)° for [4a]+; 151.7(4) and 151.5(4)° for the two independent molecules of [4b]+; to be compared to the values observed in the 143÷157° range. The C(14)–C(15) [1.390(13) Å for [4a]+; 1.355(6) and 1.360(6) Å for the two independent molecules of [4b]+] and C(15)-C(16) [1.328(13) Å for [4a]+; 1.326(6) and 1.329(5) Å for the two independent molecules of [4b]+] bonds display considerable π-character (usual values for reported structures are in the ranges 1.36÷1.40 Å and 1.31÷1.35 Å, respectively). The bridging CO ligand is substantially asymmetric in all structures with Ru(1)–C(11) and Ru(2)–C(11) distances of 1.946 and 2.186 Å for [4a]+, 1.979 and 2.185 Å, 1.972 and 2.175 Å for the two independent molecules of [4b]+, being η1-coordinated to the allenyl ligand.
51
Figure 4: Molecular structure of [4a]+ in [4a]BPh4 with key atoms labeled [all H-atoms, except
H(14), have been omitted for clarity]. Thermal ellipsoids are at the 30% probability level. Only the main images of the two disordered Cp ligands are drawn.
Figure 5: Molecular structure of [4b]+ ]+ in [4b]BPh4 with key atoms labeled [all H-atoms,
except H(14), have been omitted for clarity]. Thermal ellipsoids are at the 30% probability level. Only one of the two independent cations present within the unit cell is represented.
52 3.2.2 Deprotonation Reactions
The chemistry of the cationic species 4 with a variety of compounds (i.e. NaH, NaBH4, KCN, lithium alkyls, lithium acetylides, alkynes, alkenes, amines, phosphines
and isocyanides) was explored. Hence, all the neutral reactants except amines did not react even at high temperature.
Otherwise, ionic reactants and amines acted as Brönsted bases towards 4b at room temperature, resulting in deprotonation reactions. Thus, the allenylidene dinuclear compound [Ru2Cp2(CO)2(µ-CO){µ-η1:η1-Cα=Cβ=Cγ(Ph)2}]4b,58 (10) was obtained by
reaction of 4b with a basei (Scheme 49) and identified by spectroscopic methods and elemental analysis. Ru Ru C O Cp Cp CO OC Cα H Cβ Cγ Ph Ph Ru Ru C O Cp Cp CO OC Cα Cβ Cγ Ph Ph NaH + 4b 10
Scheme 49: Deprotonation of complex 4b.
The IR spectrum of 10 (in CH2Cl2) , reported in Figure 6, exhibits the absorptions
related to the three CO ligands, at 1991 (vs), 1954 (s) and 1803 (s) cm−1.
i
53 2136,1 2100 2000 1950 1900 1850 1800 1750 1706,2 -2,7 10 20 30 40 50 60 70 80 90 100 110 117,7 cm-1 %T 1804 1955 1991
Figure 6: FT-IR spectrum (CH2Cl2) of complex 10.
The 1H- and 13C-NMR spectra (in CDCl3) (the 1H-NMR spectrum is reported in Figure
7) show a single resonance for the two Cp rings, coherently with the symmetry exhibited by the molecule. The allenylidene-chain carbons resonate at 192.0 (Cα), 201.4
(Cβ) and 105.8 ppm (Cγ), in agreement with what reported previously for similar
compounds59.
1991, CO
1954, CO
54 0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10
Figure 7: 1H-NMR spectrum (CDCl3) of complex 10.
3.2.3 Generation of a vacant metal site: synthesis of acetonitrile and chloride derivatives
The reaction leading to 10 evidences the presence of acidic sites within the µ-allenyl unit in the cationic complexes 4. This fact prevents the possibility to address additions of nucleophiles, which are also Brönsted bases, to the allenyl ligand. On the other hand, neutral species (e.g. alkynes, alkenes) are almost unreactive towards 4 (see above). In fact, the availability of a vacant metal site in a dinuclear compound is an essential requirement for further intramolecular coupling reactions between the incoming reactant and the bridging hydrocarbyl ligand2c,d,i,5e,60. The vacancy may be generated upon replacement of one carbonyl with a labile ligand, and acetonitrile has been often used to the purpose31.
5.31, Cp
7.70-7.25, Ph
55
Thus, the complexes [Ru2Cp2(CO)(NCMe)(µ-CO){µ-η1:η2-Cα(H)=Cβ=Cγ(R)2}][BF4]
(R = Me, 6a; R = Ph, 6b) have been prepared by reaction of an acetonitrile solution of 4a,b with trimethylaminooxide, according to the known procedure22, (Scheme 50) and have been used in situ for subsequent reactions. Displacement of the nitrile ligand by chloride ion takes place in THF solution to give the neutral [Ru2Cp2
(CO)(Cl)(µ-CO){µ-η1:η2-Cα(H)=Cβ=Cγ(R)2}] (R = Me, 11a; R = Ph, 11b), see Scheme 50: Synthesis of the acetonitrile (R=Me, 6a; R=Ph, 6b)
and chloride (R=Me, 11a; R=Ph, 11b) derivatives.. The X-Ray structure of 11a was previously determined, showing the chloride bound to the ruthenium σ-connected with Cα22.The newly synthesized 6b and 11b have been characterized spectroscopically. The
IR spectrum of 6b (in CH2Cl2), exhibits only two absorptions related to one terminal
and one bridging CO ligands, at 2008 (vs) and 1861 (s) cm–1 respectively, while the absorption related to the acetonitrile ligand has been found at 2305 cm–1. The IR spectrum of 11b (in CH2Cl2) exhibits once again only two absorptions related to one
terminal and one bridging CO, at lower frequencies respect to complex 6b [1992 (vs) and 1883 (s) cm–1, respectively], as expected for a neutral compound. The salient 1H NMR feature of 11b is represented by the CαH resonance, which falls at 10.06 ppm (in
CDCl3).
In principle, both compounds 6-11 may provide a vacant metal site: in 6, acetonitrile is a labile ligand and may be easily replaced; otherwise, the chloride ligand in 11 could be efficiently removed by silver salts62. We have found that the best reactant is AgSO3CF361.
56 Ru Ru C O Cp Cp CO OC Cα H Cβ Cγ R R + 4 Ru Ru C O Cp Cp CO MeCN Cα H Cβ Cγ R R + 6 Ru Ru C O Cp Cp CO Cl Cα H Cβ Cγ R R Me3NO MeCN LiCl - MeCN 11
Scheme 50: Synthesis of the acetonitrile (R=Me, 6a; R=Ph, 6b) and chloride (R=Me, 11a; R=Ph, 11b) derivatives.
3.2.4 Reactions of the allenyl unit with ethyldiazoacetate/amine
In consideration of the purpose to obtain novel organic fragments by functionalization of the bridging hydrocarbyl ligand, we decided to study the reactivity of the nitrile adduct 6a with ethyldiazoacetate, N2CH(CO2Et). The reaction gave the butenolide
derivative [Ru2Cp2(CO)(µ-CO){µ-η1:η3-Cα(H)CβCγ(Me)2OC(=O)C(H)] (13a), which
was isolated as a crystalline solid after work-up; by following analogous procedure, we
prepared the bis-phenyl analogues
[Ru2Cp2(CO)(µ-CO){µ-η1:η3-Cα(H)CβCγ(Ph)2OC(=O)C(H)] (13b), see Scheme 51. The reaction leading to 13a proceeds smoothly in dichloromethane solution at room temperature, and it was monitored by IR spectroscopy. Progressive consumption of the starting metal compound was observed, whereas two new carbonylic bands appeared at 1980 and 1812 cm−1. These were attributed to the cationic adduct
57
[Ru2Cp2(CO)(µ-CO){µ-η1:η3-Cα(H)CβCγ(Me)2OC(OEt)C(H)]+ (12). After few hours, new IR bands came along at lower wavenumbers (1963, 1792 and 1735 cm−1), suggesting the conversion of 12 into 13a by removal of a [Et]+ unit (Scheme 51). Compound 12 could not be isolated in the solid state, however it was further characterized by a 1H NMR experiment (see Experimental). We have observed that the addition of a molar excess of NEt3, forcing the abstraction of the Et+ fragment, makes
the conversion 12 → 13a quantitative.
Ru Ru C O Cp Cp CO MeCN Cα H Cβ Cγ R R + 6a,b Ru Ru C O Cp Cp OC Cα H Cβ Cγ R R C C O O H Ru Ru C O Cp Cp OC Cα H Cβ Cγ Me Me C C O OEt H + 12 13a R=Me 13b R=Ph N2CHCO2Et - MeCN - N2 NEt3 - EtOH N2CHCO2Et/NEt3
58
Scheme 51: Reaction steps for the synthesis of heterocycle-substituted carbene ligands via reactions of the allenyl unit with ethyldiazoacetate/amine
The new complexes 13a,b have been fully characterized by IR and NMR spectroscopy, elemental analysis and X-Ray diffraction.
3.2.5 Spectroscopic characterization of the products
The IR spectra of 13a,b (CH2Cl2) show two bands due to a terminal carbonyl ligand and
a bridging one (e.g. for 13b at 1964 and 1795 cm–1), and another absorption related to the acyl group at ca. 1740 cm–1 (see Figure 8), in agreement with the presence of an ester unit. 2113,2 2000 1950 1900 1850 1800 1750 1700 1647,6 34,0 40 45 50 55 60 65 70 75 80 85 90 95 98,4 cm-1 %T 1964 1794 1746
Figure 8: FT-IR spectrum (CH2Cl2) of complex 13b in the carbonyl region.
1964, CO
1795, µ−CO
59
The salient NMR features are represented by the CαH resonances [e.g. for 13a: δ(1H)
= 10.04 ppm; δ(13C) = 137.9 ppm]. Coupling along the Cα–C–C(Ο) chain is evident in
the 1H NMR spectrum of 13a, where CαH and CH(O) protons resonate as doublets (4JHH
= 1.47 Hz). Furthermore, the 13C NMR resonances of the Cβ and of the CβCC(O) carbon
atoms fall at ca. 115 and 40 ppm respectively, while the 13C resonance of the lactone C=O carbon is seen at ca. 180 ppm (See Figure 9 and 10).
0 0 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8 9 9 10 10 11 11
Figure 9: 1H-NMR (CDCl3) spectrum of complex 13b.
10.35, CαH
7.59-7.33, Ph
5.37, 4.55, Cp
1.59, CH
60 25 25 50 50 75 75 100 100 125 125 150 150 175 175 200 200 225 225 250 250
Figure 10: 13C-NMR (CDCl3) spectrum of complex 13b.
3.2.6 X-ray Structures
The ORTEP representations of complexes 13a,b are shown in Figure 11 and Figure 12. 238.0 µ−CO 201.2, CO 178.3, C=O 141.6, CαH 112.4, Cβ 89.2, 85.4, Cp 61.5, Cγ 43.8, CH 145.0-125.9, Ph ppm
61
Figure 11: Molecular structure of 13a. The H-atoms, except H(14) and H(15), have been omitted for clarity. Thermal ellipsoids are at the 30% probability level.
Figure 12: Molecular structure of 13b. The H-atoms, except H(13) and H(15), have been omitted for clarity. Thermal ellipsoids are a the 30% probability level.
62
The molecular structures of 13a,b consist of a bridging 5-dimethyl(diphenyl)-2-furanone-4-carbene ligand [µ-η1:η3-C(H)CC(R2)OC(=O)C(H)] coordinated to the cis-[Ru2Cp2(CO)(µ-CO)] core. The carbene ligand possesses partial bridging allylidene
character, hence two main resonance formulas may be traced for its representation (see Scheme 52: Resonance formulas for compounds 13.)62. Indeed both C(13)–C(14) [1.406(2) Å and 1.404(6) Å for 13a and 13b, respectively] and C(14)–C(15) interactions [1.433(2) Å and 1.411(7) Å] show appreciable π-character. Conversely, the C(15)–C(16) [1.461(3) Å and 1.487(7) Å] and C(14)–C(17) [1.533(2) Å and 1.527(6) Å] are characteristic for Csp2–Csp2 and Csp2–Csp3 single bonds. Similarly, C(16)–O(1) [1.205(2) Å and 1.205(6)
Å] is a C=O double bond, whereas both C(16)–O(2) [1.376(3) Å and 1.358(6) Å] and C(17)–O(2) [1.461(2) Å and 1.479(6) Å] are in agreement with Csp2–O and Csp3–O
single bonds. The C(14)–C(15)–C(16)–O(2)–C(17) ring is almost planar [mean deviations from the least squares plane are 0.0469 and 0.0494 Å, respectively].
Ru Ru C O Cp Cp OC Cα H Cβ Cγ R R C C O O H Ru Ru C O Cp Cp OC Cα H Cβ Cγ R R C C O O H
![Figure 5: Molecular structure of [4b] + ] + in [4b]BPh 4 with key atoms labeled [all H-atoms,](https://thumb-eu.123doks.com/thumbv2/123dokorg/4786936.48683/51.918.323.632.582.871/figure-molecular-structure-bph-key-atoms-labeled-atoms.webp)
![Figure 13: Structure of the cation 14 in [14][BF 4 ]·CH 2 Cl 2 . The H-atoms, except H(13) and](https://thumb-eu.123doks.com/thumbv2/123dokorg/4786936.48683/65.918.299.679.318.593/figure-structure-cation-bf-ch-cl-h-atoms.webp)
